Enamines, restricted rotation

N-chemical shifts of enamines and closely related amines have been determined using 15N of natural abundance62. In order to eliminate substituent effects, differential chemical shifts A<5(N) are defined as <5N(amine)-<5N(enamine). This parameter is shown to correlate well with the free energy of activation AG for restricted rotation about the N—C bond of enamines with extended conjugation. The experimental results suggest that the differential 15N shifts are a useful probe to study n-n interactions in enamines. 

The other systems, esters 2.71, enamines 2.72, and enol ethers 2.73, similarly have restricted rotation about the bond drawn as a single bond, but the barrier is... 

Development in the area of asymmetric aza-annulation reactions paralleled achievements in the analogous area of asymmetric Michael addition reactions with chiral imines.111 Induction of asymmetry has been primarily controlled through substitution at the nitrogen of the imine or enamine that becomes incorporated into the heterocycle. Restricted rotation of this asymmetric substituent led to preferred conformational isomers, which provided stereofacial bias for carbon-carbon bond formation. 

Similarly, aza-annulation with an acyclic substrate resulted in a high degree of stereocontrol. These results suggested that intramolecular hydrogen bonding of the intermediate enamine controlled the enamine geometry and served to restrict rotation of the chiral auxiliary (eq. 103).116 In this case, 507 was sensitive to hydrolysis, and isolation was performed after hydrolysis to 508. 

In the figure, the (S)- isomer is shown with the upper phosphorus atom behind the plane of the paper and the lower one in front of it. These are reversed in the (R) + form and the two molecules are not superimposable on their mirror images. Construction of molecular models will clearly demonstrate both the restricted rotation around the bond between the two naphthalene rings and also the non-superimposability of the two enantiomers. One form adds the hydrogen atom to one face of the enamine and the other adds it to the opposite one. Thus, using the (5) + form of BINAP, only af-citronellal is produced. 

At the same time, rotation about the formally single bond between N-l and C-2 in these compounds is more restricted than the drawing of a single bond implies, just as it was with amides. The two A-methyl groups in both enamines 2.63 and 2.82 have different chemical shifts and coalescence measurements show that the free energy of activation for rotation is 56 kJ mol 1 (13 kcal mol-1) for the former and 69 kJ mol-1 (16.5 kcal mol-1) for the latter. Decreasing the stabilisation of the anionic centre in the transition structure with a less powerful acceptor than a nitro group, as in the ester 2.83 reduces the barrier to rotation about the N—C bond to 58 kJ mol-1 (14 kcal mol-1). 

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